It may mean that you were once angry at your sister but thought you have gone too far with your temper. Just because you kill someone in a dream does not mean your a killer in real life, its just the irrational part of your subconscious called the id that is driven by sexual lust and aggression thats inhibited by the ego and the superego.

Killing and death is dreams is not what it appears to be. Dreams are the language of the subconscious and as such symbols are used to express other things. So death or killing in a dream could simply mean that you are ready to see the "end" of a relationship or a situation. Maybe your sister is not even the focus of the dream. Sometimes characteristics of aperson can be represented through the imagery of someone you know. Your sister could be representing yourself even. A part of you don't like. Don't take it literal. It doesn't mean you want to kill your sister. Good luck)